1. Field of the Invention
The present invention relates to an image processing method and image processing apparatus for processing data to be printed with pigment ink.
2. Description of the Related Art
Some printing apparatuses such as a printer, copying machine, and facsimile apparatus are configured to print an image formed from a dot pattern on a printing medium such as paper or a thin plastic plate based on image information. These printing apparatuses are classified by printing method into an inkjet type, wire dot type, thermal type, laser beam type, and the like. Of these printing apparatuses, an inkjet type (inkjet printing apparatus) prints by discharging ink (printing liquid) droplets from the orifices of a printhead onto a printing medium. The inkjet printing apparatus can perform non-contact printing by discharging ink from the printhead and can stably print images on a wide variety of printing media. The inkjet printing apparatus has received attention as a printing apparatus which can meet demands for quick printing, high resolution, high image quality, low noise, and the like.
For ink, pigment ink and dye ink are used. In particular, pigment ink is superior in weather resistance to dye ink and is even recently used in inkjet printing apparatus. Weather resistance includes light resistance, ozone resistance, and water resistance. As a feature, the pigment particle hardly loses color development upon decomposition by light or ozone. Since no color fades even upon long-term exposure to light or ozone, pigment ink exhibits excellent performance when used for outdoor advertisements or exhibitions that are to be displayed over a long period or inkjet photographs that are to be saved for a long term. The pigment particle is water-insoluble and is superior in water resistance to the dye ink. The pigment ink is widely used for general printed materials.
As for the photographic image quality in the inkjet printing apparatus, a quality representation cannot be obtained without a rich gamut at a dark portion. Increasing the ink discharging amount can achieve a high black OD (Optical Density) but without a rich saturation in a dark region. In contrast, increasing the ink density can obtain satisfactory saturation but impairs the reliability of the discharge characteristic. Hence, the black OD and the gamut in a dark region cannot be improved at the same time.
In general, the pigment ink has a property in which the pigment remains on the surface when a pigment ink of a single color is used for glossy paper. When the glossy paper is covered with the pigment, the gloss of the pigment itself becomes dominant and intensifies reflection of light. Here, attention is paid to the fact that the glossiness (light reflection amount) of a pigment black (Bk) ink tends to be high. The OD of the Bk ink generally becomes higher for weaker light reflection, so higher glossiness of the Bk ink results in a lower black OD. To improve the black OD, it is important to lower the glossiness of the Bk ink and suppress reflection of light.
In Japanese Patent Laid-Open Nos. 2002-307755 and 2005-052984, the glossiness is controlled using a clear ink or the like containing no pigment. In Japanese Patent Laid-Open No. 2002-307755, nonuniformity of glossiness between a region where a color ink is printed and a region where no color ink is printed is reduced by printing a clear ink in the region where no color ink is printed. In Japanese Patent Laid-Open No. 2005-052984, a large amount of clear ink is printed in a region where the color ink amount is small. The region where the color ink amount is small is lower in glossiness than a region where the color ink amount is large. For this reason, a large amount of clear ink is printed in the region where the color ink amount is small, increasing the glossiness and reducing nonuniformity of the glossiness within a single image.
However, in these patent literatures, the clear ink is applied by a small amount or is not applied in a region where the discharging amount of a color ink such as Bk ink is large. Hence, no glossiness can be decreased around the darkest portion where the Bk ink discharging amount is large.